1. Field of Invention
The invention relates to power takeoff (PTO) drive systems for utility vehicles and, in particular, for agricultural tractors.
2. Description of Related Art
For many years agricultural tractors have been fitted with PTO systems which allow the transfer of torque from the prime mover to an attached implement. Examples of implements that utilise PTO systems include balers, fertilizer spreaders, seed drills and hedge cutters to name but a few.
Some implements, tillage implements for example, are power hungry and place a constant high load upon the PTO typically operating at the nominal engine speed to deliver the optimum efficiency. Other implements such as fertilizers spreaders place a low load upon the PTO but still demand the maximum speed. The demanded PTO speed dictates the speed at which the engine must be run due to the direct mechanical connection between the engine and the output PTO stub. In the case of low load at applications this results in excessive fuel consumption and noise.
Some implements demand a constant ratio between the groundspeed and the PTO speed. Some tractors provide a groundspeed PTO mode wherein the propulsion drive to the wheels is directly coupled by meshed gears to the PTO stub. The ratio between groundspeed and PTO speed in such a mode is fixed by the gears installed during manufacture and the size of tyre fitted.
Where a groundspeed PTO mode is provided, problems may occur when switching between groundspeed and engine speed PTO drive. For example, where the groundspeed PTO is connected to the front or rear axle driveline, the PTO cannot be switched under load and issues such as interlocking between tractor and implement during turns cannot be avoided.
But engaging the groundspeed PTO under load may be advantageous when a tractor and a trailer with driven axle is operated under changing conditions so that the driver can engage the driven trailer axle without stopping the vehicle. E. g. a tractor with a hauling container is used to unload a forage harvester. When the hauling trailer gets heavier due to loaded crops, the driver can engage the driven trailer axle without having to stop the vehicle during overloading.
It is an object of the invention to provide a PTO drive system for a utility vehicle such as an agricultural tractor which at least alleviates some the aforementioned problems.